The present invention relates to piston-cylinder units, such as gas springs, in which the length of the working stroke of the piston can be adjusted continuously (infinitely) within a predetermined range of adjustment.
There are numerous practical applications for piston-cylinder units in which it is advantageous to be able to adjust the length of the working stroke of the piston (hereinafter xe2x80x9cstroke lengthxe2x80x9d), such as to adapt a standard piston/cylinder unit for use in different specific installation conditions. In principle, the stroke length can be established by special valve designs of the piston-cylinder unit. In this regard, reference may be made to DE 33 01 544 A1. In a piston-cylinder unit of the type disclosed in DE 33 01 544 A1, the piston rod automatically travels out to a specific stopping point and can then, by overcoming a blocking valve, arbitrarily assume a new stopping position in a second opening region. Generally, the function is reliably performed. A disadvantage of that approach is that the special valve makes the product more expensive to produce.
In a piston-cylinder unit according to either DE 40 22 145 A1 or DE 38 26 862 A1, the stroke length adjustment can be set by way of a stop sleeve having several stop faces located at different positions axially of the cylinder. The piston rod, together with the piston, is rotated relative to the cylinder of the piston-cylinder unit, so that the piston rests on a desired stop face that defines a stroke length. A disadvantage of multiple stop faces is that only a limited number of stroke lengths are possible, inasmuch as the size of each stop face must be kept large enough to avoid compression contact stresses in excess of a strength limit.
An object of the present invention is to provide a piston-cylinder unit in which the stroke length is adjustable continuously throughout a predetermined range of adjustment and that can be manufactured at relatively low cost, is reliable in operation, is easy to adjust, and is durable.
The foregoing object is attained, according to the invention, by a piston-cylinder unit having an adjustable stroke length that includes a cylinder having an axis, a closed end and a rod end, a piston rod supported in the rod end of the cylinder in sealed and sliding relation, and a piston received on the piston rod for axial movement relative to the piston rod and received in the cylinder for axial movement relative to the cylinder. A threaded connection is provided between the piston rod and the piston. A stop surface on the side of the piston facing the rod end of the cylinder and a counterstop surface associated in fixed relation with the cylinder and facing the closed end of the cylinder are engaged when the rod is extended from the cylinder to a full stroke length position. Upon such engagement the piston is held rotationally fixed to the cylinder such that upon rotation of the cylinder about the axis, the piston is rotated relative to the piston rod and is moved axially along the piston rod by the threaded connection such as to change the stroke length.
The threaded connection between the piston and piston rod allows for simple and inexpensive manufacture, provides reliable operation for stroke length adjustment, and permits continuous (infinite) adjustments of the stroke length within the range allowed by the length of the threaded portion of the piston rod.
In preferred embodiments, the stop and the counterstop have interlocking profile surfaces, such as corrugations or teeth, that are engageable with one another such as to inhibit rotation of the piston relative to the cylinder upon adjustment of the stroke length.
The threaded connection between the piston and the piston rod may be provided by external threads formed directly on the piston rod and internal threads formed directly on a portion of a body of the piston. Alternatively, the threaded connection is provided by external threads on a sleeve affixed to a body of the piston and external threads on a flange affixed to the piston rod. In the latter case, the body of the piston has an opening through which the piston rod is received and by which the piston is guided along the piston rod upon adjustment of the stroke length. Advantageously, a seal is provided between the opening in the piston and the piston rod.
In one arrangement for damping the outward movement of the piston rod at the end of the outward working stroke, a limit cup is received in the rod end of the cylinder, and a displacer is provided on the piston that moves into the limit cup near the end of the outward stroke of the piston rod. The damping effect at the end of the working stroke is the same, regardless of the adjusted stroke length. The counterstop may be associated with the limit cup.
According to other refinements of embodiments of the present invention, the cylinder may have at least one bypass groove, which connects the two working spaces separated by the piston as a function of piston position. In association with the stroke length adjustment feature according to the present invention, the characteristics of the bypass remain the same, regardless of the stroke length setting. For example, the piston movement may be damped near the end of the outward travel, and such damping is available and uniform for all stroke lengths.
It is known that the spring force characteristic of gas springs may be favorably altered to attain a desired load profile by the use of one or more compression springs within the cylinder. For this purpose, a piston-cylinder unit according to the present invention may include an axially fixed support ring on the end of the piston rod nearer the closed end of the cylinder and a mechanical compression spring engaged between the closed end of the cylinder and the support ring and applying a force to the piston rod regardless of the stroke length setting.